IBM ThinkPad R40 2681 Hardware Maintenance Manual page 115

Removing and replacing a FRU
For machine type 2684, 2685
v System board without IBM security chip
Begin by checking the FRU number of the system board,
and be sure to use the correct system board FRU for the
computer. For the FRU numbers, see "Parts list" on page
134.
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ThinkPad R40